Multi-Carrier (MC) High Speed Downlink Packet Access (HSDPA) transmission was standardized in 3GPP Rel-8/9/10/11. This allows a wireless User Equipment (UE) to simultaneously receive data transmissions from multiple cells. For MC-HSDPA, it is required that all cells, on which the downlink transmission occurs to a particular UE, belong to the same sector and have identical cell timing, i.e., are time-aligned. This allows the use of one High Speed Dedicated Physical Control Channel (HS-DPCCH) carrying the Hybrid Automatic Repeat reQuest ACKnowledgement (HARQ-ACK) and Pre-Coding Information/Channel Quality Information (PCI/CQI) feedback for all cells, without compromising the HARQ-ARQ time budget for the NodeB or UE.
At the RAN#53 plenary, a work item on multi-flow (MF) HSDPA was initiated, as detailed in RP-111375, “HSDPA Multiflow Data Transmissions,” the disclosure of which is incorporated by reference herein in its entirety. MF-HSDPA transmissions allow a UE to receive data from different, potentially uncoordinated, cells. Further, during the RAN1#66bis meeting, an agreement was made that MF-HSDPA transmission should be supported in combination with 2×2 MIMO.
For MF-HSDPA, data is spread across two or more sectors. One HS-DPCCH design alternative is to let the UE pair the HS-PDSCH Transmission Time Intervals (TTI), resulting in maximum overlap, and jointly encode the HARQ-ACK information. Such a solution was discussed during the study item, as detailed in TR 25.872, “High Speed Packet Access (HSDPA) multipoint transmission, v 11.0.0,” the disclosure of which is incorporated herein by reference in its entirety. However, this approach does not support inter-site scenarios, where Multiple Input, Multiple Output (MIMO) is configured on one or more of the cells. This is because the 3GPP Rel-9 Dual-Cell HSDPA with MIMO codebook assumes that the receiver knows the number of High Speed Downlink Shared CHannel (HS-DSCH) packets that were transmitted.
The Technical Standard 3GPP TS 25.212 specifies, in Table 15C.2 in subsection 4.7.3B.1, the channel coding for the composite HS-DPCCH HARQ-ACK, when the UE is configured for MIMO mode and a secondary cell is enabled. In this Table, presented in FIG. 1 for ease of reference, the feedback related to the serving HS-DSCH cell is given before the divider sign and the feedback related to the secondary serving HS-DSCH cell is given after the divider sign. ‘A’ means ‘ACK’, ‘N’ means ‘NACK’ and ‘D’ means ‘no transmission’ (‘DTX’). ‘AA’, ‘AN’, ‘NA’ and ‘NN’ refer to feedback for dual-stream transmission in a cell. For example, ‘AN’ means ACK on the primary stream and NACK on the secondary stream. The coding of Table 15C.2 is also referred to herein as the “Rel-9 codebook.”
The coding of Table 15C.2 assumes that the receiver knows the number of HS-DSCH packets that were transmitted. For example, by inspection of Table 15C.2, one can see that the same codeword is used for ANN and NA/NN. Without knowledge of the number of transmitted blocks on the secondary serving cell, there is an uncertainty in the decoding of the primary serving cell.
Another drawback of a solution based on joint HARQ-ACK encoding is the decrease in the HARQ time budget at the UE and/or NodeB. If cells with different cell timings are jointly encoded, this reduction can be up to 1.5 slots. This reduction has to be taken by NodeB and/or UE. Also, new events might need to be introduced to account for potential timing drifts of the clocks at the two sectors. This may, for example, require non-trivial intervention by the RNC, which is not desirable. Furthermore, these type of events will increase the RNC load.